1. Field of the Invention
The present invention relates to an acoustic diaphragm used in a loudspeaker and others and a method for manufacturing an acoustic diaphragm.
2. Description of Related Art
Requirements of a diaphragm for loudspeaker having excellent reproduction frequency characteristics are such that the Young's modulus and internal loss be large and the density be low. For example, the reproduction frequency band can be extended by increasing the Young's modulus of a diaphragm, and the reproduction frequency characteristics can be flattened by increasing the internal loss of the diaphragm to lower the quality factor. In addition, the reproduction efficiency can be improved by lowering the density of the diaphragm. For meeting the requirements of the diaphragm, the use of a so-called mica paper multi-cellular structure product in a diaphragm for loudspeaker has been proposed wherein the mica paper multi-cellular structure product is prepared by making paper from fine mica flakes and pulp fibers or polyvinyl alcohol fibers and then heating the paper to form a multi-cellular structure.
The thus prepared mica paper multi-cellular structure product has an apparent density as small as 0.05 to 0.60 g/cm3 and hence can achieve a lightweight diaphragm. Further, the fine mica flakes used in the multi-cellular structure product have a large ratio of the area to the thickness (that is, a so-called aspect ratio is large), and therefore the areas with which the mica flakes are stacked on one another are large. For this reason, the mica paper multi-cellular structure product has a large Young's modulus.
However, this multi-cellular structure product has a small internal loss and hence increases the resonance sharpness, making it difficult to obtain flat reproduction frequency characteristics. For solving this problem, a method has been proposed in Examined Japanese Patent Application Publication (KOKOKU) No. 7-28476 (Patent Document 1), in which the mica paper multi-cellular structure product is impregnated with a synthetic resin solution or synthetic resin emulsion to be coated with an extremely thin resin film, lowering the internal loss